Among the various types of devices for monitoring and/or therapeutically stimulating a patient's heart, a great many are known which are responsive to detected cardiac activity to operate in a particular manner. Perhaps the most common example of such a device is the so-called demand pacemaker, which is responsive to sensed electrical cardiac activity in either or both chambers of the heart, such that pacing pulses are delivered only in the absence of intrinsic cardiac activity. Implantable, cardiac defibrillators and cardioverters are also typically made responsive to sensed electrical cardiac activity.
For any device in which proper operation depends upon sensing and interpreting cardiac electrogram (EGM) signals, it is of course desirable to enable the device to accurately interpret the signals and to correctly identify the various types of electrical events, including atrial beats (P-waves), ventricular beats (QRS complexes), and so on, which may occur. For example, it is common in demand pacemakers for various timing intervals to be started or stopped based upon detection of ventricular activity. Thus, a sensed ventricular event might inhibit a ventricular pacing output and initiate a base rate timer, a V-A escape interval timer, and an upper rate limit timer, while a sensed atrial event might inhibit an atrial pacing output and initiate an A-V delay interval timer. Operation of the pacemaker then depends upon whether certain expected events occur, and, if so, when such events occur in relation to the various time intervals established by the pacemaker.
While certain types of EGM events may be quite readily detected with a fair degree of certainty, there are certain other types of EGM events which are susceptible to being misinterpreted, misidentified (i.e., a false positive indication), or undetected (i.e., a false negative indication). As manifested in an EGM signal, viewed, for example, on a cathode ray tube or paper strip chart, one event may have only very subtle morphological differences from another, making discrimination and identification of either of the events quite difficult. The misidentification of, or inability to detect specific types of EGM events can have such undesirable consequences as preventing a therapeutic device from properly treating the heart, or improperly allowing the device to deliver a therapy when none is actually needed.
The ability to distinguish different morphologies would be useful in a number different situations, such as when it is necessary to discriminate between normal (antegrade) and abnormal (retrograde) activation of the atria. Such ability would be useful in preventing pacemaker mediated tachycardia (PMT), in which retrograde P waves or far-field R-waves cause continuous A-V sequential pacing at a high rate. At the present time, PMT is typically prevented by programming a prolonged post-ventricular atrial refractory period (PVARP) so that the retrograde P wave or far-field R-wave is not "seen" by the atrial channel; however, this limits the upper tracking rate of the pacemaker. Alternatively, various algorithms have been developed to terminate PMT once it is established. The ability to distinguish antegrade from retrograde P waves or far-field R-waves would be a far better solution to the problem of PMT. In this situation, retrograde P waves or far-field R-waves would be ignored by the pacemaker, abolishing the potential for PMT.
Another situation in which the ability to distinguish between the subtly different morphologies of certain types of EGM events is when it is necessary to discriminate between normal ventricular activation and abnormal ventricular activation. This would be useful in premature ventricular contraction (PVC) detection as well as detection of ventricular arrhythmias, such as ventricular tachycardia. Current generations of defibrillators use only a rate criteria to distinguish between normal beats and arrhythmias; the ability to distinguish between normal and abnormal EGM morphologies would be a significant improvement.
Various methods have been proposed in the prior art for distinguishing between different electrogram morphologies. In Wainwright et. al., "Ideal Atrial Lead Positioning to Detect Retrograde Atrial Depolarization by Digitization and Slope Analysis of the Atrial Electrogram ", PACE, 7:1152-1158, (1984), for example, there is proposed a digital system for examining changes in slew rates of the atrial EGM signals recorded from the right atrial appendage, high right atrium, and low right atrium, in order to distinguish antegrade from retrograde P waves. However, the signal processing capabilities described by Wainwright appear to be beyond that currently available in implantable devices.
Davies et. al., "Detection of Pathological Tachycardia by Analysis of Electrogram Morphology", PACE, 9:200-208, (1986), discussed the possibility of using the method described by Wainwright et al. to distinguish normal from abnormal ventricular activation. In Panizzo et. al., "Discrimination of Antegrade and Retrograde Atrial Depolarization by Electrogram Analysis", American Heart Journal, 112:780-786 (1986), measurement of EGM amplitudes and slew rates was proposed to distinguish antegrade from retrograde atrial activation. Panizzo et al. apparently concluded that combining a magnitude and slew rate threshold allowed discrimination of antegrade from retrograde P waves in 34 of 34 cases examined.
Timmis et. al., "Discrimination of Antegrade from Retrograde Atrial Electrograms for Physiologic Pacing", PACE, 11:130-140 (1988) discussed the examination of multiple EGM parameters, including peak-to-peak amplitude, duration, energy, maximum slew rate, mean slew rate, and polarity in the time domain, and maximum frequency, half-power frequency, Fourier amplitude peak, and frequency of peak in the frequency domain, and apparently concluded that no single parameter reliably distinguished antegrade from retrograde atrial activation, although the use of multiple parameters could be useful.
McAlister et. al., "Atrial Electrogram Analysis: Antegrade Versus Retrograde", PACE, 11: 1703-1707 (1988) suggested examination of morphology, slew rate, and amplitude criteria in discriminating antegrade from retrograde P waves, and seemed to indicate that morphology and slew rate did not increase the discriminatory power over amplitude alone. Lin et. al., "Identification of Ventricular Tachycardia Using Intracavitary Ventricular Electrograms: Analysis of Time and Frequency Domain Patterns", PACE, 11:1592-1606 proposed the use of sophisticated signal processing techniques, including correlation analysis and spectral analysis, in distinguishing ventricular tachycardia from sinus tachycardia, and suggested that the correlation waveform analysis is a reliable technique which requires significant data processing resources.
In view of the foregoing, it appears to the inventor that it would be desirable to provide morphologic analysis capability in an implantable device, in order to improve the device's ability to accurately detect and respond to various classes of EGM events.